Plasma arc torches are widely used in the high temperature processing (e.g., cutting, welding, and marking) of metallic materials. As shown in FIG. 1A, a plasma arc torch generally includes a torch body 1, an electrode 2 mounted within the body, an insert 3 disposed within a bore of the electrode 2, a nozzle 4 with a central exit orifice, a shield 5, electrical connections (not shown), passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply (not shown). The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. A gas can be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air.
In all plasma arc torches, particularly those using a reactive plasma gas, the electrode shows wear over time in the form of a generally concave pit at the exposed emission surface of the insert. The pit is formed due to the loss of molten high emissivity material from the insert. The emission surface liquefies when the arc is generated, and electrons are emitted from a molten pool of the high emissivity material during the operation of the electrode. However, the molten material is ejected from the emission surface during torch operation.
In the process of plasma arc cutting or marking a metallic workpiece, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). The pilot arc ionizes gas that passes through the nozzle exit orifice. After the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc is then transferred from the nozzle to the workpiece. Generally the torch is operated in this transferred plasma arc mode, which is characterized by the flow of ionized plasma gas from the electrode to the workpiece, for the cutting, welding, or marking the workpiece.
Copper electrode bodies with an insert of high thermionic emissivity material are used in a plasma arc torch with a reactive plasma gas. FIG. 1B illustrates a known method for inserting and securing an insert into the bore of an electrode. FIG. 1B illustrates an emissive insert 10 being pressed 15 into a bore 17 in the end of an electrode body 12, resulting in a press fit electrode insert.
During the operation of plasma arc torch electrodes, torch conditions such as temperature gradients and dynamics work to reduce the retention force holding the insert in place and either allow the insert to move in the bore 17 or to fall completely out of the bore, thereby reducing the service life of the electrode. As described in U.S. Pat. No. 8,101,882 to Hypertherm, Inc., the entire contents of which are incorporated herein by reference, electrodes and inserts can be configured to retain inserts without movement. However, as the plasma arc torch thermally cycles the electrode, retention forces typically degrade and eventually allow some movement of the insert. Further, as described in U.S. Pat. Nos. 5,310,988 and 6,130,399 to Hypertherm, Inc., the insert material melts at the tip of the electrode during operation, creating a pit in the exposed end of the insert. Pit growth occurs until the electrode fails completely, e.g., when the arc emitted from the emissive insert of the electrode attaches to the front copper portion of the electrode body.
What is a needed is an electrode with improved cutting life for a plasma arc torch that better utilizes the insert material within the cavity of the electrode, during operation of the torch.